The 3rd-Generation Partnership Project (3GPP) is continuing development of the fourth-generation wireless network technologies known as Long-Term Evolution (LTE). Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-10, and further improvements are being discussed in the context of new features for Release-11. In heterogeneous networks, a mixture of cells of different sizes and overlapping coverage areas are deployed.
One example of such a deployment is seen in the system 100 illustrated my FIG. 1, where several pico-cells, each comprising a base station 120 or low-power transmitting/receiving node with a respective coverage area 150, are deployed within the larger coverage area 140 of a macro-cell, which comprises a base station 110 or high-power transmitting/receiving node. The terms node or point refers to a base station of any kind. The system 100 of FIG. 1 is suggestive of a wide-area wireless network deployment. However, other examples of low-power nodes in heterogeneous networks are home base stations and relays. As will be discussed in further detail below, the large difference in output power, e.g., 46 dBm in macro cells and 30 dBm or less in pico cells, results in different interference scenarios from those that are seen in networks where all base stations have the same output power.
Throughout this document, nodes or points in a network are often referred to as being of a certain type, e.g., a “macro” node, or a “pico” point. However, unless explicitly stated otherwise, this should not be interpreted as an absolute quantification of the role of the node or point in the network but rather as a convenient way of discussing the roles of different nodes or points relative to one another. Thus, a discussion about macro- and pico-cells could just as well be applicable to the interaction between micro-cells and femto-cells, for example.
One aim of deploying low-power nodes such as pico base stations within the macro coverage area is to improve system capacity, by means of cell-splitting gains. In addition to improving overall system capacity, this approach also allows users to be provided with a wide-area experience of very-high-speed data access, throughout the network. Heterogeneous deployments are in particular effective to cover traffic hotspots, i.e., small geographical areas with high user densities. These areas can be served by pico cells, for example, as an alternative deployment to a denser macro network.
The most basic means to operate heterogeneous networks is to apply frequency separation between the different layers. For instance, the macro-cell 110 and pico-cells 120 pictured in FIG. 1 can be configured to operate on different, non-overlapping carrier frequencies, thus avoiding any interference between the layers. With no macro-cell interference towards the under-laid cells, i.e., the cells having coverage areas falling substantially or entirely within the coverage area of the macro-cell, cell-splitting gains are achieved when all resources can simultaneously be used by the under-laid cells.
One drawback of operating layers on different carrier frequencies is that it may lead to inefficiencies in resource utilization. For example, if there is a low level of activity in the pico-cells, it could be more efficient to use all carrier frequencies in the macro-cell, and then basically switch off the pico-cells. However, the split of carrier frequencies across layers in this basic configuration is typically done in a static manner.
Another approach to operating a heterogeneous network is to share radio resources between layers. Thus, two or more layers can use the same carrier frequencies, by coordinating transmissions across macro- and under-laid cells, such as pico or femto cells. This type of coordination is referred to as inter-cell interference coordination (ICIC). With this approach, certain radio resources are allocated to the macro cells for a given time period, whereas the remaining resources can be accessed by the under-laid cells without interference from the macro cell. Depending on the traffic situations across the layers, this resource split can change over time to accommodate different traffic demands. In contrast to the earlier described static allocation of carrier frequencies, this way of sharing radio resources across layers can be made more or less dynamic depending on the implementation of the interface between the nodes. In LTE, for example, an X2 interface has been specified in order to exchange different types of information between base station nodes, for coordination of resources. One example of such information exchange is that a base station can inform other base stations that it will reduce transmit power on certain resources.
Time synchronization between base station nodes is generally required to ensure that ICIC across layers will work efficiently in heterogeneous networks. This is of particular importance for time-domain-based ICIC schemes, where resources are shared in time on the same carrier.
Orthogonal Frequency-Division Multiplexing (OFDM) technology is a key underlying component of LTE. OFDM is a digital multi-carrier modulation scheme employing a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is separately modulated using conventional modulation techniques and channel coding schemes. In particular, 3GPP has specified Orthogonal Frequency Division Multiple Access (OFDMA) for the downlink transmissions from the base station to a mobile terminal, and single carrier frequency division multiple access (SC-FDMA) for uplink transmissions from a mobile terminal to a base station. Both multiple access schemes permit the available sub-carriers to be allocated among several users.
SC-FDMA technology employs specially formed OFDM signals, and is therefore often called “pre-coded OFDM” or Discrete-Fourier-Transform (DFT)-spread OFDM. Although similar in many respects to conventional OFDMA technology, SC-FDMA signals offer a reduced peak-to-average power ratio (PAPR) compared to OFDMA signals, thus allowing transmitter power amplifiers to be operated more efficiently. This in turn facilitates more efficient usage of a mobile terminal's limited battery resources. SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMA for Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine, vol. 1, no. 3, September 2006, pp. 30-38.
The basic LTE physical resource can be seen as a time-frequency grid. This concept is illustrated in FIG. 2, which shows a number of so-called subcarriers in the frequency domain, at a frequency spacing of Δf, divided into OFDM symbol intervals in the time domain. Each individual element of the resource grid 210 is called a resource element 220, and corresponds to one subcarrier during one OFDM symbol interval, on a given antenna port. One aspect of OFDM is that each symbol 230 begins with a cyclic prefix 240, which is essentially a reproduction of the last portion of the symbol 230 affixed to the beginning. This feature minimizes problems from multipath propagation, over a wide range of radio signal environments.
In the time domain, LTE downlink transmissions are organized into radio frames of ten milliseconds each, each radio frame consisting of ten equally-sized subframes of one millisecond duration. This is illustrated in FIG. 3, where an LTE signal 310 includes several frames 320, each of which is divided into ten subframes 330. Not shown in FIG. 3 is that each subframe 330 is further divided into two slots, each of which is 0.5 milliseconds in duration.
LTE link resources are organized into “resource blocks,” defined as time-frequency blocks with a duration of 0.5 milliseconds, corresponding to one slot, and encompassing a bandwidth of 180 kHz, corresponding to 12 contiguous sub-carriers with a spacing of 15 kHz. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Two time-consecutive resource blocks represent a resource block pair, and correspond to the time interval upon which scheduling operates. Of course, the exact definition of a resource block may vary between LTE and similar systems, and the inventive methods and apparatus described herein are not limited to the numbers used herein.
In general, however, resource blocks may be dynamically assigned to mobile terminals, and may be assigned independently for the uplink and the downlink. Depending on a mobile terminal's data throughput needs, the system resources allocated to it may be increased by allocating resource blocks across several sub-frames, or across several frequency blocks, or both. Thus, the instantaneous bandwidth allocated to a mobile terminal in a scheduling process may be dynamically adapted to respond to changing conditions.
For scheduling of downlink data, the base station transmits control information in each subframe. This control information identifies the mobile terminals to which data is targeted and the resource blocks, in the current downlink subframe, that are carrying the data for each terminal. The first one, two, three, or four OFDM symbols in each subframe are used to carry this control signaling. In FIG. 4, a downlink subframe 410 is shown, with three OFDM symbols allocated to control region 420. The control region 420 consists primarily of control data elements 434, but also includes a number of reference symbols 432, used by the receiving station to measure channel conditions. These reference symbols 432 are interspersed at pre-determined locations throughout the control region 420 and among the data symbols 436 in the data portion 430 of the subframe 410.
Transmissions in LTE are dynamically scheduled in each subframe, where the base station transmits downlink assignments/uplink grants to certain mobile terminals, e g user equipments (UEs), in 3GPP terminology, via the physical downlink control channel (PDCCH). The PDCCHs are transmitted in the control region of the OFDM signal, i.e., in the first OFDM symbol(s) of each subframe, and span all or almost all of the entire system bandwidth. A UE that has decoded a downlink assignment, carried by a PDCCH, knows which resource elements in the subframe that contain data aimed for that particular UE. Similarly, upon receiving an uplink grant, the UE knows which time-frequency resources it should transmit upon. In the LTE downlink, data is carried by the physical downlink shared channel (PDSCH) and in the uplink the corresponding channel for carrying data is referred to as the physical uplink shared channel (PUSCH).
LTE also employs multiple modulation formats, including at least QPSK, 16-QAM, and 64-QAM, as well as advanced coding techniques, so that data throughput may be optimized for any of a variety of signal conditions. Depending on the signal conditions and the desired data rate, a suitable combination of modulation format, coding scheme, and bandwidth is chosen, generally to maximize the system throughput. Power control is also employed to ensure acceptable bit error rates while minimizing interference between cells. In addition, LTE uses a hybrid-ARQ (HARQ) error correction protocol where, after receiving downlink data in a subframe, the terminal attempts to decode it and reports to the base station whether the decoding was successful (ACK) or not (NACK). In the event of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
Demodulation of transmitted data generally requires estimation of the radio channel. In LTE systems, this is done using transmitted reference signals (RS), i.e., signals comprising symbols having values that are already known to the receiver. In LTE, cell-specific reference signals (CRS) are transmitted in all downlink subframes. In addition to assisting downlink channel estimation, the CRS are also used for mobility measurements performed by the UEs.
The CRS are generally intended for use by all the mobile terminals in the coverage area. To support improved channel estimation, especially when multiple-input multiple-output (MIMO) transmission techniques are used, LTE also supports UE-specific reference signals, which are targeted to individual mobile terminals and are intended specifically for channel estimation for demodulation purposes.
FIG. 4 illustrates how the mapping of physical control/data channels and signals can be done on resource elements within a downlink subframe 410. In the pictured example, the PDCCHs occupy only the first out of the three possible OFDM symbols that make up the control region 420, so in this particular case the mapping of data can begin at the second OFDM symbol. Since the CRS are common to all UEs in the cell, the transmission of CRS cannot be easily adapted to suit the needs of a particular UE. This is in contrast to UE-specific reference signals, by means of which each UE can have reference signals of its own placed in the data region 430 of FIG. 4, as part of PDSCH.
The length of the control region that is used to carry PDCCH, e.g., one, two, or three symbols, can vary on a subframe-to-subframe basis, and is signaled to the UE in the Physical Control Format Indicator CHannel (PCFICH). The PCFICH is transmitted within the control region, at locations known by terminals. Once a terminal has decoded the PCFICH, it then knows the size of the control region and in which OFDM symbol the data transmission starts.
Also transmitted in the control region is the Physical Hybrid-ARQ Indicator Channel. This channel carries ACK/NACK responses to a terminal, to inform the mobile terminal whether the uplink data transmission in a previous subframe was successfully decoded by the base station.
As noted above, CRS are not the only reference signals available in LTE. As of LTE Release-10, a new reference signal concept was introduced. Separate UE-specific reference signals for demodulation of PDSCH are supported in Release 10, as are reference signals specifically provided for measuring the channel for the purpose of generating channel state information (CSI) feedback from the UE. The latter reference signals are referred to as CSI-RS. CSI-RS are not transmitted in every subframe, and they are generally sparser in time and frequency than reference signals used for demodulation. CSI-RS transmissions may take place every fifth, tenth, twentieth, fortieth, or eightieth subframe, as determined by a periodicity parameter and a subframe offset, each of which are configured by Radio Resource Control (RRC) signaling.
A UE operating in connected mode can be requested by the base station to perform channel state information (CSI) reporting. This reporting can include, for example, reporting a suitable rank indicator (RI) and one or more precoding matrix indices (PM's), given the observed channel conditions, as well as a channel quality indicator (CQI). Other types of CSI are also conceivable, including explicit channel feedback and interference covariance feedback. The CSI feedback assists the base station in scheduling, including deciding which subframe and resource blocks to use for the transmission, as well as deciding which transmission scheme and/or precoder should be used. The CSI feedback also provides information that can be used to determine a proper user bit-rate for the transmission, i.e., for link adaptation.
In LTE, both periodic and aperiodic CSI reporting are supported. In the case of periodic CSI reporting, the terminal reports the CSI measurements on a configured periodic time basis, using the physical uplink control channel (PUCCH). With aperiodic reporting, the CSI feedback is transmitted on the physical uplink shared channel (PUSCH) at pre-specified time instants after receiving the CSI grant from the base station. With aperiodic CSI reports, the base station can thus request CSI that reflects downlink radio conditions in a particular subframe.
A detailed illustration of which resource elements within a resource block pair that may potentially be occupied by the new UE-specific reference signals (DMRS) and CSI-RS is provided in FIG. 5, for the cases in which two, four, and eight transmitter antenna ports are used for the CSI transmission. The CSI-RS utilizes an orthogonal cover code of length two to overlay two antenna ports on two consecutive resource elements. In other words, the CSI-RS are allocated in pairs, where two orthogonal codes of length two are transmitted simultaneously, using the same pair of allocated resource elements, from a pair of antenna ports at the base station.
In FIG. 5, the CSI-RS resource elements are designated with numbers, which correspond to antenna port numbers. In the left-hand diagram, corresponding to the case of two CSI-RS antenna ports, the possible positions for the CSI-RS are labeled “0” and “1”, corresponding to antenna ports 0 and 1.
As can be seen in FIG. 5, many different CSI-RS patterns are available. For the case of two CSI-RS antenna ports, for instance, where each CSI-RS pair can be separately configured, there are twenty different patterns within a subframe. When there are four CSI-RS antenna ports, the CSI-RS pairs are assigned two at a time; thus the number of possible patterns is ten. For the case of eight CSI-RS antenna ports, five patterns are available. For TDD mode, some additional CSI-RS patterns are available.
In the following discussion, the term “CSI-RS resource” is used. A CSI-RS resource corresponds to a particular pattern present in a particular subframe. Thus two different patterns in the same subframe constitute two distinct CSI-RSI resources. Likewise, the application of the same CSI-RS pattern to two different subframes again represents two separate instances of a CSI-RS resource, and the two instances are thus again to be considered distinct CSI-RS resources.
Any of the various CSI-RS patterns pictured in FIG. 5 may also correspond to so-called zero-power CSI-RS, which are also referred to as muted REs. A zero-power CSI-RS is a CSI-RS pattern whose resource elements are silent, i.e., there is no transmitted signal on those resource elements. These silent patterns are configured with a resolution corresponding to the four-antenna-port CSI-RS patterns. Hence, the smallest unit of silence that may be configured corresponds to four REs.
The purpose of zero-power CSI-RS is to raise the signal-to-interference-plus-noise ratio (SINR) for CSI-RS in a given cell, by configuring zero-power CSI-RS in interfering cells so that the resource elements that would otherwise cause interference are silent. Thus, a CSI-RS pattern in a given cell is matched with a corresponding zero-power CSI-RS pattern in interfering cells.
Raising the SINR level for CSI-RS measurements is particularly important in applications such as coordinated multi point (CoMP) or in heterogeneous deployments. In CoMP, the UE is likely to need to measure the channel from non-serving cells. Interference from the much stronger serving cell would make those measurements difficult, if not impossible. Zero-power CSI-RS are also needed in heterogeneous deployments, where zero-power CSI-RS in the macro-layer are configured to coincide with CSI-RS transmissions in the pico-layer. This avoids strong interference from macro nodes when UEs measure the channel to a pico-node.
The PDSCH, which carries data targeted for mobile stations, is mapped around the resource elements occupied by CSI-RS and zero-power CSI-RS, so it is important that both the network and the UE are assuming the same CSI-RS and zero power CSI-RS configurations. Otherwise, the UE may be unable to properly decode the PDSCH in subframes that contain CSI-RS or their zero-power counterparts.
The CSI-RS discussed above are used for measurements of the downlink channel, i.e., from a base station to a mobile terminal. In the uplink, so-called sounding reference signals (SRS) may be used for acquiring CSI about the uplink channel from the UE to a receiving node. When SRS are used, they are transmitted on the last DFT-spread OFDM symbol of a subframe. SRS can be configured for periodic transmission as well for dynamic triggering as part of the uplink grant. The primary use for SRS is to aid the scheduling and link adaptation in the uplink. For time-division duplex (TDD) LTE systems, however, SRS are sometimes used to determine beam-forming weights for the downlink, by exploiting the fact that the downlink and uplink channels are the same when the same carrier frequency is used for downlink and uplink, i e there is channel reciprocity.
While PUSCH carries data in the uplink, PUCCH is used for control. PUCCH is a narrowband channel using a resource block pair where the two resource blocks are on opposite sides of the potential scheduling bandwidth. PUCCH is used for conveying ACK/NACKs, periodic CSI feedback, and scheduling request to the network.
Before an LTE terminal can communicate with an LTE network it first has to find and acquire synchronization to a cell within the network, a process known as cell search. Next, the UE has to receive and decode system information needed to communicate with and operate properly within the cell. Finally, the UE can access the cell by means of the so-called random-access procedure.
In order to support mobility, a terminal needs to continuously search for, synchronize to, and estimate the reception quality of both its serving cell and neighbor cells. The reception quality of the neighbor cells, in relation to the reception quality of the current cell, is then evaluated in order to determine whether a handover, for terminals in connected mode, or cell re-selection, for terminals in idle mode, should be carried out. For terminals in connected mode, the handover decision is taken by the network, based on measurement reports provided by the terminals. Examples of such reports are reference signal received power (RSRP) and reference signal received quality (RSRQ).
The results of these measurements, which are possibly complemented by a configurable offset, can be used in several ways. The UE can, for example, be connected to the cell with the strongest received power. Alternatively, the UE can be assigned to the cell with the best path gain. An approach somewhere between these alternatives may be used.
These selection strategies do not always result in the same selected cell for any given set of circumstances, since the base station output powers of cells of different type are different. This is sometimes referred to as link imbalance. For example, the output power of a pico base station or a relay node is often on the order of 30 dBm (1 watt) or less, while a macro base station can have an output power of 46 dBm (40 watts). Consequently, even in the proximity of the pico cell, the downlink signal strength from the macro cell can be larger than that of the pico cell. From a downlink perspective, it is often better to select a cell based on downlink received power, whereas from an uplink perspective, it would be better to select a cell based on the path loss.
These alternative cell selection approaches are illustrated in FIG. 6. The solid lines emanating from each of macro-cell 110 and pico-cell 120 represent the received power at each point between the two cells. These lines intersect, i.e., are equal, at border 540. Accordingly, a UE within region 510 will see a stronger received signal from the pico-cell 120, and will get the best downlink performance if it selects pico-cell 120. The dashed lines issuing from pico-cell 120 and macro-cell 110, on the other hand, represent the path loss between a UE at a given point and either the macro-cell 110 or the pico-cell 120. Because the path loss is not weighted by the transmitter output power, these lines intersect at a point halfway between macro-cell 110 and pico-cell 120, as seen at the border 530. A UE outside region 520, then, will experience a lower path loss to macro-cell 110 than to pico-cell 120, and will thus achieve better uplink performance if it selects macro-cell 110. Because of this unbalanced situation, there is a region, i.e., the portion of coverage area 520 that is outside coverage area 510, in which neither cell is optimal for both downlink and uplink performance at the same time.
From a system perspective, it might often be better, in the above scenario, for a given UE to connect to the pico-cell 120 even under some circumstances where the downlink from macro-cell 110 is much stronger than the pico cell downlink. However, ICIC across layers will be needed when the terminal operates within the region between the uplink and downlink borders, i.e., the link imbalance zone, as depicted in FIG. 6.
Interference coordination across the cell layers is especially important for the downlink control signaling. If the interference is not handled appropriately, a terminal that is in the region between the downlink and uplink borders in FIG. 6 and is connected to pico-cell 120 may be unable to receive the downlink control signaling from the pico-cell 120.
One approach to providing ICIC across layers is illustrated in FIG. 7. An interfering macro-cell, which could create downlink interference towards a pico-cell, transmits a series of subframes 710, but avoids scheduling unicast traffic in certain subframes 712. In other words, neither PDCCHs nor PDSCH are transmitted in those subframes 712. In this way, it is possible to create low-interference subframes, which can be used to protect users of the pico-cell who are operating in the link imbalance zone.
To carry out this approach, the macro-base station (MeNB) indicates to the pico-base station (PeNB), via the backhaul interface X2, which subframes will not be used to schedule users. The PeNB can then take this information into account when scheduling users operating within the link imbalance zone, such that these users are scheduled only in subframes 722 aligned with the low-interference subframes transmitted in the macro layer. In other words, these users are scheduled only in interference-protected subframes. Pico-cell users operating within the downlink border, e.g., within coverage area 510 in FIG. 6, can be scheduled in all subframes, i.e., in both the protected subframes 722 as well as the remaining, un-protected, subframes in the series of subframes 720.
In principle, data transmission, but generally not control signaling, in different layers could also be separated in the frequency domain by ensuring that scheduling decisions in the two cell layers are non-overlapping in the frequency domain. This could be facilitated by exchanging coordination messages between the different base stations. However, this is not possible for the control signaling, since the control signaling spans the full bandwidth of the signal, according to the LTE specifications, and hence a time-domain approach must be used.
The classical way of deploying a network is for each different transmission/reception point to provide coverage for a cell that is distinct from all others. That is, the signals transmitted from or received at one point are associated with a cell identifier (cell-id) that is different from the cell-id employed for other nearby points. Typically, each of these points transmits its own unique signals for broadcast, e.g., the Physical Broadcast Channel (PBCH), as well as for sync channels, such as the primary synchronization signal (PSS) and secondary synchronization signal (SSS).
The concept of a “point” is heavily used in conjunction with techniques for coordinated multipoint (CoMP). In this context, a point corresponds to a set of antennas covering essentially the same geographical area in a similar manner. One transmitting/receiving node, such as an LTE base station, might control one or several points. Thus, a point might correspond to one of the sectors at a base station site, but it may also correspond to a site having one or more antennas all intending to cover a similar geographical area. Often, different points represent different sites. Antennas correspond to different points when they are sufficiently geographically separated and/or have antenna diagrams pointing in sufficiently different directions. Techniques for CoMP entail introducing dependencies in the scheduling or transmission/reception among different points, in contrast to conventional cellular systems where a point is operated more or less independently from the other points, from a scheduling point of view.
The classical strategy of one cell-id per point is depicted in FIG. 8 for a heterogeneous deployment where a number of low-power, e.g., pico, points 120 are placed within the coverage area of a higher power macro point 110. In this deployment, the pico-nodes transmit different cell identifiers, i.e., “cell-id 2”, “cell-id 3”, and “cell-id 4”, from the cell identifier “cell-id 1” transmitted by the macro-cell 110. Note that similar principles obviously also apply to classical macro-cellular deployments where all points have similar output power and perhaps are placed in a more regular fashion than what is the case for a heterogeneous deployment.
An alternative to the classical deployment strategy is to instead let all the UEs within a geographical area outlined by the coverage of the high-power macro point be served with signals associated with the same cell-id. In other words, from a UE perspective, the received signals appear as though they come from a single cell. This is illustrated in FIG. 9. Here, all of the pico-nodes 120 transmit the same cell identifier, “cell-id 1”, which is also used by the overlaying macro-cell 110, i.e., the cell having a coverage area that contains, at least substantially the coverage area for one or more of the pico-cells.
Note that in both FIGS. 8 and 9 only one macro point is shown; other macro points would typically use different cell-ids, e.g., corresponding to different cells, unless they are co-located at the same site, e.g., corresponding to other sectors of the macro site. In the latter case of several co-located macro points, the same cell-id may be shared across the co-located macro-points and those pico points that correspond to the union of the coverage areas of the macro points. Sync, BCH and control channels are all transmitted from the high-power point while data can be transmitted to a UE also from low-power points by using shared data transmissions (PDSCH) that rely on UE-specific reference signals.
Such an approach has benefits for those UEs that are capable of receiving PDSCH based on UE-specific reference signals, while UEs that only support CRS for PDSCH have to settle for using only the transmission from the high-power point, and thus will not benefit in the downlink from the deployment of extra low-power points. This latter group is likely to include at least all Release 8 and 9 UEs for use in frequency-division duplex (FDD) LTE systems.
The single cell-id approach for heterogeneous and/or hierarchical cell deployments is geared towards situations in which there is fast backhaul communication between the points associated with the same cell identifier. A typical case would be a base station serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) performing the role of the other points that share the same cell-id. Those RRUs could represent low-power points with one or more antennas each. Another example is when all the points have a similar power class, with no single point having more significance than the others. The base station would then handle the signals from all RRUs in a similar manner.
A clear advantage of the shared cell-id approach compared with the classical one is that the handover procedure between cells only needs to be invoked on a macro basis. Another important advantage is that interference from CRS can be greatly reduced, since CRS does not have to be transmitted from every point. There is also much greater flexibility in coordination and scheduling among the points, which means the network can avoid relying on the inflexible concept of semi-statically configured low-interference subframes, as illustrated in FIG. 7. A shared-cell approach also allows decoupling of the downlink from the uplink, so that, for example, path-loss-based reception-point selection can be performed for the uplink, without creating a severe interference problem for the downlink, where the UE may be served by a transmission point different from the point used in the uplink receptions.